1. Field of the Invention
The present invention relates generally to interbody spinal fusion implants. Implants, artificial or natural, are placed at least in part within a disc space and in contact with each of the vertebral bodies adjacent that disc space for spacing apart and aligning those vertebral bodies and for allowing for the growth of bone in continuity from vertebral body to adjacent vertebral body through said implant.
2. Description of the Related Art
Some of the degenerative conditions that affect the spine may be so severe as to require surgical intervention. For example, a disc herniation may compress the spinal cord and/or nerve roots and cause pain, loss of function, and even complete paralysis of the legs with loss of bowel and bladder control. The correct treatment for such conditions is the removal of the offending discal tissue.
Sometimes, for a variety of reasons including the removal of disc material, the spine may become unstable (too much motion) at any given level. Historically, this has been treated by fusion, the joining together permanently of the unstable vertebrae via a bridge of bone so as to eliminate all motion along that portion of the spine.
To achieve fusion, bone or bone like substances are applied between two (or more) separate and distinct bones to induce bony bridging therebetween. When such procedures are successfully performed between vertebral bodies, an interbody fusion results. The shared bone in the area previously occupied by an intervertebral disc is referred to as an interbody fusion.
When a spinal surgeon seeks to perform an interbody fusion, the spine may be accessed from a variety of directions. If the surgeon elects to approach the spine anteriorly, this generally requires severing and/or removing substantial portions of the anterior longitudinal ligament over the operated area. The anterior longitudinal ligament is positioned along the anterior spinal surface and prevents hyperextension of the spine as an individual bends backward. Because the anterior longitudinal ligament covers the anterior spinal surface, the surgeon must cut through this tough ligament to access the disc space therebelow, compromising the stability of the spine. Specifically, the anterior longitudinal ligament is generally lax, except when an individual leans backward, then the ligament acts as a tension band resisting elongation. If the anterior longitudinal ligament is damaged, there is no check on that spinal movement and the vertebral bodies may detrimentally angulate.
Without a functional anterior longitudinal ligament, the patient may damage an implant(s) placed into a disc space to facilitate interbody fusion of the adjacent vertebral bodies. The implant may crush into or erode into, the adjacent vertebral bodies as the disc space opens anteriorly and crushes down posteriorly when the patient bends backwards. The vertebrae may rock together posteriorly and open anteriorly, thus dislodging the implant. Accordingly, in at least any spinal surgery requiring access to the disc space through the anterior longitudinal ligament, there is a need to functionally reconstruct the anterior longitudinal ligament to preserve stability about the disc space to be fused. Stability of the spine across the disc space to be fused is beneficial for achieving fusion.
In order to perform anterior interbody spinal fusion, a significant amount of disc material is removed from the interspace to be fused. After removing the disc material, the disc space is filled with an implant, which generally includes bone or bone in combination with a reinforcing structure, such as an artificial (other than bone) interbody spinal fusion implant. Because of the forces and motions occurring through the spine, it is not uncommon for such implants to dislodge, thereby causing a failure of surgery and possibly warranting further surgery to correct the problem and to again attempt interbody fusion.
Metal hardware outside the disc space affixed anteriorly to the vertebral bodies adjacent the disc space to be fused is useful to ensure the stability of the spine during the fusion period. Those skilled in the art have shown great reluctance to utilize such hardware because of the potential for the hardware to impinge on vital body structures, such as the aorta, vena cava, or great iliac vessels. The rupture of any of these body structures could cause sudden death. A rupture may occur late after surgery due to the pulsing of an artery against the metal hardware resulting in the eventual erosion and rupture of the artery. Further, metal applied to the outer surfaces of the vertebral bodies may become loose. For example, a screw may back out from repeated bodily movements, leading to the above-described situation.
Therefore, there is a need for an implant that is resistant to dislodgment and functionally substitutes for the anterior longitudinal ligament at the level to be fused, without protruding from the spine.